A Radial Velocity Survey for LMC Microlensed Sources

We propose a radial velocity survey with the aim to resolve the current dispute on the LMC lensing: in the pro-macho hypothesis the lenses are halo white dwarfs or machos in general; in the pro-star hypothesis both the lenses and the sources are stars in various observed or hypothesized structures of the Magellanic Clouds and the Galaxy. Star-star lensing should prefer sources at the backside or behind the LMC disc because lensing is most efficient if the source is located a few kpc behind a dense screen of stars, here the thin disc of the LMC. This signature of self-lensing can be looked for by a radial velocity survey since kinematics of the stars at the back can be markedly different from that of the majority of stars in the cold, rapidly rotating disc of the LMC. Detailed simulations of effect together with optimal strategies of carrying out the proposed survey are reported here. Assuming that the existing 30 or so alerted stars in the LMC are truely microlensed stars, their kinematics can test the two lensing scenarios; the confidence level varies with the still very uncertain structure of the LMC. Spectroscopy of the existing sample and future events requires about two or three good-seeing nights per year at a 4m-8m class southern telescope, either during the amplification phase or long after.Comment: minor changes of text, ApJ accepte

Turbulence length scales in a vortical flow

Laser Doppler velocimetry is used to investigate the velocity spectra and turbulence length scales in a turbulent vortical flow. The turbulent vortical flow is ensured by vorticity generators (VGs) inserted into a straight circular pipe. Each VG generates a complex flow that is mainly the combination of a steady streamwise counter-rotating vortex pair and a periodic sequence of hairpin-like structures caused by the Kelvin- Helmholtz instability in the shear layer ejected from the VG trailing edges. These primary structures induce a secondary vorticity in the wake of the VG. The aim of the study is to analyze the velocity spectra and turbulent length scales for the different coherent structures in the flow. Thus, the Kolmogorov and Taylor microscales, the Liepmann-Taylor microscale and the viscous length scale are determined in different locations in the VG streamwise direction. The evolution of the length scales with respect to the Taylor-Reynolds number is compared with theoretical trends in a variety of flows in the open literature

Turbulent mixing of two immiscible fluids

International audienc

Thermal performance of High-Efficiency Vortex (HEV) variants: reversed arrays configuration

Convective heat transfer in the Reversed Arrays configuration of the High-Efficiency Vortex (HEV) multifunctional heat exchanger is investigated. An experimental test section constituted of a tube equipped with inclined trapezoidal vortex generators with a constant-flux heating system is designed and constructed. In this configuration, the tab inclination is opposite to the flow direction. Interactions between the tabs and the flow generate coherent structures in the form of longitudinal counter-rotating streamwise vortices enhancing radial particle dispersion, mixing, and ultimately heat transport. The original configuration in which the tabs are inclined in the flow direction is also examined. Recent in-house hydrodynamic and thermal studies have been conducted showing the interest of these configurations in mixing and heat transfer applications. The experimental data are in good agreement with the numerical results. Local Nusselt numbers show an increasing tendency in the longitudinal direction with remarkable cross-sectional variations. Global analysis of convective heat transfer reveals the superiority of the Reversed Arrays. Energy expenditures are assessed through total pressure drop measurements. A comparative analysis based on the thermal enhancement factor and Colburn factor shows that the HEV is energetically less costly than other heat exchangers with similar heat transfer capacity

Transport phenomena in chaotic flows: flux recombination HEX reactors

Rapid transport of heat and mass is required in many industrial processes. Mixing is a fundamental issue in chemical engineering applications and when exothermic reactions are involved, heat transfer capabilities of reactors and static mixers become an advantage and a necessity to ensure stable operating conditions and security standards. Enhancement of mixing and heat exchange is possible through turbulence, but vortical structures are often not feasible for highly viscous, non-Newtonian or shear sensitive fluids such as emulsions, pastes and slurries common in pharmaceutical, cosmetic and food industries. An alternative to improve transport within such materials is chaotic advection, where Lagrangian chaotic structures are induced by physical means in low-Reynolds laminar flows. Microfluidics is an increasingly active domain in which small dimensions and velocities render turbulent mixing extremely hard. Mixing by diffusion is one solution where topological mixing schemes exploiting the laminarity the flow to repeatedly fold the flow and exponentially increase the concentration gradients to obtain fast and efficient mixing by diffusion. This paper presents the first results of a study investigating laminar and turbulent mixing qualities of a Flux Recombination Hex reactor by using the chemical probe method. The geometry, exploiting a three-dimensional, steady flow configuration intended to mimic the baker’s map and enhance mixing by chaotic advection. First proposed by Chen & Meiners [1] for a microfluidic chip, it is here reproduced for investigation purposes using a stratified multiple plate manufacturing technique on a mini-scale where laminar and slightly turbulent regimes can be assessed

Effet des rangées de perturbateurs pariétaux sur les transferts de chaleur

L’étude numérique du transfert de chaleur dans un échangeur de type HEV (High Efficiency Vortices) permet d’expliquer les mécanismes de l’intensification induits par les perturbateurs de paroi. L’effet des différentes structures générées est ainsi mis en évidence. Les performances globales du HEV montrent qu’il affiche une meilleure efficacité énergétique par rapport à d’autres échangeurs du marché

Kinematic mixing and heat transfer enhancement in chaotic split-and-recombine heat exchangers/reactors

Small system dimensions, low fluid velocity and high viscosity are all factors that hinder the production of turbulence. Enhancing mixing and heat transfer under these conditions, while keeping sufficient residence times and moderate pressure drops, constitutes a real challenge. Adapted to low-Reynolds flow regimes, Split-And-Recombine (SAR) static mixer and heat exchanger configurations are designed to exploit flow energy to produce chaotic advection and promote diffusion at the molecular level. The present work explores the hydrodynamic and thermal character of the SAR flow and compares, through CFD simulations, two such geometries namely SAR-1 and SAR-2, with two other reference configurations: a square three-dimensional continuous flow geometry (3D-Flow) and a plain square channel. Efficient convective heat transfer is achieved in deeply laminar creeping flow. Relative enhancements up to 1700% can be achieved compared to plain square channel flow, with a moderate increase in the pressure drop that does not exceed 17% for the SAR-2 configuration showing the better performance

On the synergy field between velocity vector and temperature gradient in turbulent vortical flows

The intensity of the secondary flow induced, especially, by streamwise vorticity, which are generated in their turn by vortex generators or in flows with curved streamlines has a direct impact on the heat transfer process. Thus the understanding and quantification of the physical mechanisms underlying the heat transfer by streamwise vorticity are fundamental for practical applications such as multifunctional heat exchangers/reactors (MHER) used in chemical processing industry, cooling of electronic systems and data centers, as well as biomedical engineering. In the present study, CFD simulations are performed to investigate the synergy field in two different flows. The synergy field principle is based on the assertion that the included angles θ between the streamlines and the isotherms is related to the heat flux that arises. From the local distribution of the intersection angle in the flow cross section, it is found that in the thinning region of the thermal boundary layer where the Nusselt number is the highest, θ is minimum. By introducing a characteristic parameter defined as the volume-averaged θ, it is found that the lowest θ value corresponds to the flow configuration presenting the highest Nusselt number. This confirms that the transport phenomena are intensified in the flow where the geometry minimizes this parameter. Finally, the study discusses the use of the synergy field principle in three dimensional turbulent vortical flows, and presents a new intensified MHER which can be used in several industrial processes